Load control device for a light-emitting diode light source

ABSTRACT

A load control device for controlling the intensity of a lighting load, such as a light-emitting diode (LED) light source, may include a power converter circuit operable to receive a rectified AC voltage and to generate a DC bus voltage, a load regulation circuit operable to receive the bus voltage and to control the magnitude of a load current conducted through the lighting load, and a control circuit operatively coupled to the load regulation circuit for pulse width modulating or pulse frequency modulating the load current to control the intensity of the lighting load to a target intensity. The control circuit may control the intensity of the lighting load by pulse width modulating the load current when the target intensity is above a predetermined threshold and control the intensity of the lighting load by pulse frequency modulating the load current when the target intensity is below the predetermined threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 14/290,584,filed May 29, 2014, which claims the benefit of U.S. Provisional PatentApplication No. 61/828,337, filed May 29, 2013, the contents of whichare hereby incorporated by reference in their entirety.

BACKGROUND

Light-emitting diode (LED) light sources are often used in place of oras replacements for conventional incandescent, fluorescent, or halogenlamps, and the like. LED light sources may comprise a plurality oflight-emitting diodes mounted on a single structure and provided in asuitable housing. LED light sources are typically more efficient andprovide longer operational lives as compared to incandescent,fluorescent, and halogen lamps. In order to illuminate properly, an LEDdriver control device (i.e., an LED driver) may be coupled between apower source (e.g., an alternating-current (AC) source) and the LEDlight source for regulating the power supplied to the LED light source.The LED driver may regulate either the voltage provided to the LED lightsource to a particular value, the current supplied to the LED lightsource to a specific peak current value, or may regulate both thecurrent and voltage.

LED light sources may comprise a plurality of individual LEDs that maybe arranged in a series and parallel relationship. In other words, aplurality of LEDs may be arranged in a series string and a number ofseries strings may be arranged in parallel to achieve the desired lightoutput. For example, five LEDs in a first series string each with aforward bias of approximately three volts (V) and each consumingapproximately one watt of power (at 350 mA through the string) consumeabout 5 W. A second string of a series of five LEDs connected inparallel across the first string will result in a power consumption of10 W with each string drawing 350 mA. Thus, an LED driver would supply700 mA to the two strings of LEDs, and since each string has five LEDs,the output voltage provided by the LED driver would be about 15 volts.Additional strings of LEDs can be placed in parallel for additionallight output, however, the LED driver should be operable to provide thenecessary current. Alternatively, more LEDs can be placed in series oneach string, and as a result, the LED driver should also be operable toprovide the necessary voltage (e.g., 18 volts for a series of six LEDs).

LED light sources are typically rated to be driven via one of twodifferent control techniques: a current load control technique or avoltage load control technique. An LED light source that is rated forthe current load control technique is also characterized by a ratedcurrent (e.g., 350 milliamps) to which the peak magnitude of the currentthrough the LED light source should be regulated to ensure that the LEDlight source is illuminated to the appropriate intensity and color. Incontrast, an LED light source that is rated for the voltage load controltechnique is characterized by a rated voltage (e.g., 15 volts) to whichthe voltage across the LED light source should be regulated to ensureproper operation of the LED light source. Typically, each string of LEDsin an LED light source rated for the voltage load control techniqueincludes a current balance regulation element to ensure that each of theparallel legs has the same impedance so that the same current is drawnin each parallel string.

In addition, it is known that the light output of an LED light sourcecan be dimmed. Different methods of dimming LEDs include a pulse-widthmodulation (PWM) technique and a constant current reduction (CCR)technique. Pulse-width modulation dimming can be used for LED lightsources that are controlled in either a current or voltage load controlmode. In pulse-width modulation dimming, a pulsed signal with a varyingduty cycle is supplied to the LED light source. If an LED light sourceis being controlled using the current load control technique, the peakcurrent supplied to the LED light source is kept constant during an ontime of the duty cycle of the pulsed signal. However, as the duty cycleof the pulsed signal varies, the average current supplied to the LEDlight source also varies, thereby varying the intensity of the lightoutput of the LED light source. If the LED light source is beingcontrolled using the voltage load control technique, the voltagesupplied to the LED light source is kept constant during the on time ofthe duty cycle of the pulsed signal in order to achieve the desiredtarget voltage level, and the duty cycle of the load voltage is variedin order to adjust the intensity of the light output. Constant currentreduction dimming is typically only used when an LED light source isbeing controlled using the current load control technique. In constantcurrent reduction dimming, current is continuously provided to the LEDlight source, however, the DC magnitude of the current provided to theLED light source is varied to thus adjust the intensity of the lightoutput.

However, an LED light source may become instable or exhibit undesirablecharacteristics when dimmed to a low intensity level or when dimmed tooff (i.e., 0% intensity). For example, when dimmed to a low intensitylevel or off, an LED light source may flicker, may exhibit inconsistentbrightness or color across the individual LEDs of the LED light source,and/or may suddenly drop in intensity during the dimming procedure(e.g., from approximately 1% to off). For instance, when dimming an LEDlight source using the PWM technique, the on time of the duty cycle ofthe pulsed signal may reach a threshold where, if reduced any further,causes the LED light source to become instable or exhibit undesirablecharacteristics. Similarly, when dimming an LED light source using theCCR technique, the DC magnitude of the current provided to the LED lightsource may reach a threshold where, if reduced any further, causes theLED light source to become instable or exhibit undesirablecharacteristics.

SUMMARY

As described herein, a load control device for controlling an intensityof a lighting load may include a load regulation circuit and a controlcircuit. The load regulation circuit may be configured to control amagnitude of a load current conducted through the light load. Thecontrol circuit may be operatively coupled to the load regulationcircuit to pulse width modulate or pulse frequency modulate the loadcurrent so that the intensity of the lighting load may be controlled toa target intensity. More specifically, when the target intensity isabove a predetermined threshold, the control circuit may pulse widthmodulate the load current to control the intensity of the lighting load.When the target intensity is below a predetermined threshold, thecontrol circuit may pulse frequency modulate the load current to controlthe intensity of the lighting load. Further, when the target intensityis below a minimum intensity that is below the predetermined threshold,the control circuit may pulse width modulate the load current to controlthe intensity of the lighting load. For example, the control circuit maycontrol the intensity of the lighting load from the minimum intensity tooff by pulse width modulating the load current. As described herein,pulse width modulating the load current may involve maintaining afrequency of the load current constant and adjusting an on time of theload current. Pulse frequency modulating the load current may involvemaintaining the on time of the load current constant and adjusting thefrequency of the load current.

The load control device described herein may further include a powerconverter circuit configured to receive a rectified AC voltage and togenerate a DC bus voltage. The load regulation circuit may be configuredto receive the DC bus voltage while the control circuit may beconfigured to maintain both a frequency of the load current and an ontime of the load current constant. The control circuit may be furtherconfigured to decrease a magnitude of the DC bus voltage when the targetintensity is below a minimum intensity that is below the predeterminedthreshold.

Also described herein is a method for controlling an intensity of alight load. The method may include a controlling step, a receiving step,a pulse width modulation step, and a pulse frequency modulation step.The controlling step may involve controlling a magnitude of a loadcurrent conducted through the lighting load. The receiving step mayinvolve receiving a target intensity for the lighting load. When thetarget intensity is above a predetermined threshold, the load currentmay be pulse width modulated at the pulse width modulation step tocontrol the intensity of the lighting load to the target intensity. Whenthe target intensity is below the predetermined threshold, the loadcurrent may be pulse frequency modulated at the pulse frequencymodulation step to control the intensity of the lighting load to thetarget intensity. Further, when the target intensity is below a minimumintensity that is below the predetermined threshold, the load currentmay be pulse width modulated at the pulse width modulation step tocontrol the intensity of the lighting load to the target intensity. Forexample, the load current may be pulse width modulated to control theintensity of the lighting load from the minimum intensity to off.

As described herein, pulse width modulating the load current may involvemaintaining a frequency of the load current constant and adjusting an ontime of the load current. The frequency may be maintained at a normalpulse width modulation (PWM) frequency while the on time may be adjustedbetween a maximum on time and a minimum on time. On the other hand,pulse frequency modulating the load current may involve maintaining theon time of the load current constant and adjusting the frequency of theload current. The on time may be maintained at the minimum on time whilethe frequency may be adjusted between the normal PWM frequency and aminimum PWM frequency.

The method for controlling an intensity of a light load may furtherinclude receiving a rectified AC voltage and generating a DC bus voltagefrom the rectified AC voltage. The DC bus voltage may be used to controlthe magnitude of the load current conducted through the lighting load.Further, the method described herein may include maintaining both afrequency of the load current and an on time of the load currentconstant, and decreasing a magnitude of the DC bus voltage when thetarget intensity is below a minimum intensity (e.g., such as a minimumintensity that is below the predetermined threshold).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example system that comprises alight-emitting diode (LED) driver for controlling the intensity of anLED light source.

FIG. 2 is a block diagram of an example of an LED driver for controllingthe intensity of an LED light source.

FIG. 3A is a schematic diagram of an example of a flyback converter andan LED drive circuit.

FIG. 3B is a schematic diagram showing an example the LED drive circuitof FIG. 3A.

FIG. 4A is a graph that illustrates an example of the relationshipbetween an on-time T_(ON) of a load current of an LED driver and atarget lighting intensity L_(TRGT) of an LED light source.

FIG. 4B is a graph that illustrates an example of the relationshipbetween a frequency f_(LOAD) of a load current of an LED driver and atarget lighting intensity L_(TRGT) of an LED light source.

FIG. 5A is a graph that illustrates an example of the relationshipbetween an on-time T_(ON) of a load current of an LED driver and atarget lighting intensity L_(TRGT) of an LED light source.

FIG. 5B is a graph that illustrates an example of the relationshipbetween a frequency f_(LOAD) of a load current of an LED driver and atarget lighting intensity L_(TRGT) of an LED light source.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of an example system that comprises alight-emitting diode (LED) driver for controlling the intensity of anLED light source. A system 108 may comprise an alternating-current (AC)power source 104, a dimmer switch 106, an LED driver 100, and/or an LEDlight source 102. The LED driver 100 may control an intensity of the LEDlight source 102. An example of the LED light source 102 may be an LEDlight engine. The LED light source 102 is shown as a plurality of LEDsconnected in series but may comprise a single LED or a plurality of LEDsconnected in series, parallel, or a suitable combination thereof, forexample, depending on the particular lighting system. The LED lightsource 102 may comprise one or more organic light-emitting diodes(OLEDs).

The LED driver 100 may be coupled to the AC power source 104 via thedimmer switch 106. The dimmer switch 106 may generate a phase-controlsignal V_(PC) (e.g., a dimmed-hot voltage). The dimmer switch 106 mayprovide the phase-control signal V_(PC) to the LED driver 100. Thedimmer switch 106 may comprise a bidirectional semiconductor switch (notshown), such as, for example, a triac or two anti-series-connectedfield-effect transistors (FETs), which may be coupled in series betweenthe AC power source 104 and the LED driver 100. The dimmer switch 106may control the bidirectional semiconductor switch to be conductive fora conduction period T_(CON) each half-cycle of the AC power source 104to generate the phase-control signal V_(PC).

The LED driver 100 may turn the LED light source 102 on and off inresponse to the conduction period T_(CON) of the phase-control signalV_(PC) received from the dimmer switch 106. The LED driver 100 mayadjust (i.e., dim) a present intensity L_(PRES) of the LED light source102 to a target intensity L_(TRGT) in response to the phase-controlsignal V_(PC). The target intensity L_(TRGT) may range across a dimmingrange of the LED light source 102. For example, the dimming range of theLED light source 102 may be between a low-end intensity L_(LE)(e.g.,approximately 1%) and a high-end intensity L_(HE) (e.g., approximately100%). The LED driver 100 may control the magnitude of a load currentI_(LOAD) through the LED light source 102 and/or the magnitude of a loadvoltage V_(LOAD) across the LED light source. Accordingly, the LEDdriver 100 may control at least one of the load voltage V_(LOAD) acrossthe LED light source 102 and the load current I_(LOAD) through the LEDlight source to control the amount of power delivered to the LED lightsource, for example, depending upon a mode of operation of the LEDdriver (e.g., as described herein).

The LED driver 100 may work with (i.e., control) a plurality ofdifferent LED light sources. For example, the LED driver 100 may workwith LED lights sources that are rated to operate using different loadcontrol techniques, different dimming techniques, and/or differentmagnitudes of load current and/or voltage. The LED driver 100 maycontrol the magnitude of the load current I_(LOAD) through the LED lightsource 102 and/or the load voltage V_(LOAD) across the LED light sourceusing different modes of operation. For example, the LED driver 100 mayuse a current load control mode (i.e., for using the current loadcontrol technique) and/or a voltage load control mode (i.e., for usingthe voltage load control technique). The LED driver 100 may adjust themagnitude to which the LED driver 100 controls the load current I_(LOAD)through the LED light source 102 in the current load control mode. TheLED driver 100 may adjust the magnitude to which the LED driver 100controls the load voltage V_(LOAD) across the LED light source in thevoltage load control mode.

When operating in the current load control mode, the LED driver 100 maycontrol the intensity of the LED light source 102 using a PWM dimmingmode (i.e., for using the PWM dimming technique), a CCR dimming mode(i.e., for using the CCR dimming technique), and/or a pulse frequencymodulation (PFM) dimming mode (i.e., for using the PFM dimmingtechnique). In the PWM dimming mode, the LED driver 100 may control theload current I_(LOAD) by altering the pulse duration of the load currentI_(LOAD) and maintaining the frequency of the load current I_(LOAD)constant. In the PFM dimming mode, the LED driver 100 may control theload current I_(LOAD) by maintaining the pulse duration of the loadcurrent I_(LOAD) constant and altering the frequency of the load currentI_(LOAD). In the CCR dimming mode, the LED driver 100 may control theload current I_(LOAD) by altering the DC magnitude of the current loadcurrent I_(LOAD). When operating in the voltage load control mode, theLED driver 100 may control the amount of power delivered to the LEDlight source 102 using the PWM dimming mode and/or the PFM dimming mode.The LED driver 100 may control the amount of power delivered to the LEDlight source 102 in response to a digital message, which may be receivedfrom a communication circuit, for example as described herein.

FIG. 2 is a block diagram of an example of an LED driver for controllingan LED light source. An LED driver 200 may comprise a radio-frequency(RFI) filter and rectifier circuit 215, a buck-boost flyback converter220, a bus capacitor C_(BUS), an LED drive circuit 230, a controlcircuit 240, a power supply 250, a phase-control input circuit 260,memory 270, and/or a communication circuit 280. The LED driver 200 maybe an example of the LED driver 100 of FIG. 1. As such, the LED driver200 may be used within the system 108 of FIG. 1. The LED driver 200 maycontrol an LED light source, such as the LED light source 102.

The RFI filter and rectifier circuit 215 may receive the phase-controlsignal V_(PC) from a dimmer switch (e.g., the dimmer switch 106 of FIG.1). The RFI filter and rectifier circuit 215 may minimize the noiseprovided on an AC power source (e.g., the AC power source 104 of FIG.1). The RFI filter and rectifier circuit 215 may generate a rectifiedvoltage V_(RECT). The buck-boost flyback converter 220 may receive therectified voltage V_(RECT). The buck-boost flyback converter 220 maygenerate a variable direct-current (DC) bus voltage V_(BUS) across thebus capacitor C_(BUS). The buck-boost flyback converter 220 may provideelectrical isolation between the AC power source and the LED lightsource 102. The buck-boost flyback converter 220 may operate as a powerfactor correction (PFC) circuit to adjust the power factor of the LEDdriver 200 towards a power factor of one. The buck-boost flybackconverter 220 may be a power converter circuit. Although illustrated asthe buck-boost flyback converter 220, the LED driver 200 may compriseany suitable power converter circuit for generating an appropriate busvoltage V_(BUS), such as, for example, a boost converter, a buckconverter, a single-ended primary-inductor converter (SEPIC), a Ćukconverter, or other suitable power converter circuit. The bus voltageV_(BUS) may be characterized by some voltage ripple as the bus capacitorC_(BUS) periodically charges and discharges.

The LED drive circuit 230 may be a load regulation circuit. The LEDdrive circuit 230 may receive the bus voltage V_(BUS). The LED drivecircuit 230 may control the amount of power delivered to the LED lightsource 102 so as to control the intensity of the LED light source 102.The LED drive circuit 230 may comprise a controllable-impedance circuit,such as a linear regulator, for example, as described herein. The LEDdrive circuit 230 may comprise a switching regulator, such as a buckconverter for example. Examples of various embodiments of LED drivecircuits 230 are described in U.S. patent application Ser. No.12/813,908, filed Jun. 11, 2010, entitled LOAD CONTROL DEVICE FOR ALIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosure of which ishereby incorporated by reference.

The control circuit 240 may control the operation of the buck-boostflyback converter 220 and/or the LED drive circuit 230. The controlcircuit 240 may comprise, for example, a controller or any othersuitable processing device, such as, for example, a microcontroller, aprogrammable logic device (PLD), a microprocessor, an applicationspecific integrated circuit (ASIC), or a field-programmable gate array(FPGA). The power supply 250 may receive the rectified voltage V_(RECT).The power supply 250 may generate a plurality of direct-current (DC)supply voltages for powering the circuitry of the LED driver 200, forexample, using the rectified voltage V_(RECT). For example, the powersupply 250 may generate a first non-isolated supply voltage V_(CC1)(e.g., approximately 14 volts) for powering the control circuitry of thebuck-boost flyback converter 220, a second isolated supply voltageV_(CC2) (e.g., approximately 9 volts) for powering the control circuitryof the LED drive circuit 230, and/or a third non-isolated supply voltageV_(CC3) (e.g., approximately 5 volts) for powering the control circuit240.

The control circuit 240 may be coupled to the phase-control inputcircuit 260. The phase-control input circuit 260 may generate a targetintensity control signal V_(TRGT). The target intensity control signalV_(TRGT) may comprise, for example, a square-wave signal having a dutycycle DC_(TRGT), which may be dependent upon the conduction periodT_(CON) of the phase-control signal V_(PC) received from a dimmer switch(e.g., the dimmer switch 106 of FIG. 1). The duty cycle DC_(TRGT) may berepresentative of the target intensity L_(TRGT) of the LED light source102. The target intensity control signal V_(TRGT) may comprise a DCvoltage having a magnitude dependent upon the conduction period T_(CON)of the phase-control signal V_(PC), and thus representative of thetarget intensity L_(TRGT) of the LED light source 102.

The control circuit 240 may be coupled to the memory 270. The memory 270may store the operational characteristics of the LED driver 200 (e.g.,the load control mode, the dimming mode, the magnitude of the rated loadvoltage or current, and/or the like). The communication circuit 280 maybe coupled to, for example, a wired communication link or a wirelesscommunication link, such as a radio-frequency (RF) communication link oran infrared (IR) communication link. The control circuit 240 may updatethe target intensity L_(TRGT) of the LED light source 102 and/or theoperational characteristics stored in the memory 270 in response todigital messages received via the communication circuit 280. Forexample, the LED driver 200 may receive a full conduction AC waveformfrom the AC power source (i.e., not the phase-control signal V_(PC) fromthe dimmer switch) and may determine the target intensity L_(TRGT) forthe LED light source 102 from the digital messages received via thecommunication circuit 280.

The control circuit 240 may manage the operation of the buck-boostflyback converter 220 and/or the LED drive circuit 230 to control theintensity of the LED light source 102. The control circuit 240 mayreceive a bus voltage feedback signal V_(BUS-FB), which may berepresentative of the magnitude of the bus voltage V_(BUS), from thebuck-boost flyback converter 220. The control circuit 240 may provide abus voltage control signal V_(BUS-CNTL) to the buck-boost flybackconverter 220 for controlling the magnitude of the bus voltage V_(BUS)to a target bus voltage V_(BUS-TRGT) (e.g., from approximately 8 voltsto 60 volts). The LED drive circuit 230 may control a peak magnitudeI_(PK) of the load current I_(LOAD) conducted through the LED lightsource 102 between a minimum load current I_(LOAD-MIN) and a maximumload current I_(LOAD-MAX) (e.g., when operating in the current loadcontrol mode), for example, in response to a peak current control signalV_(IPK)provided by the control circuit 240. The control circuit 240 mayreceive a load current feedback signal V_(ILOAD), which isrepresentative of an average magnitude I_(AVE) of the load currentI_(LOAD) flowing through the LED light source 102. The control circuit240 may receive a regulator voltage feedback signal V_(REG-FB), which isrepresentative of the magnitude of a regulator voltage V_(REG) (i.e., acontrollable-impedance voltage) across the linear regulator of the LEDdrive circuit 230, for example, as described herein.

The control circuit 240 may control the LED drive circuit 230 to controlthe amount of power delivered to the LED light source 102 using thecurrent load control mode of operation and/or the voltage load controlmode of operation. During the current load control mode, the LED drivecircuit 230 may regulate the peak magnitude I_(PK) of the load currentI_(LOAD) through the LED light source 102 to control the averagemagnitude I_(AVE) to a target load current I_(TRGT) in response to theload current feedback signal V_(ILOAD) (i.e., using closed loopcontrol). The target load current I_(TRGT) may be stored in the memory270. The target load current I_(TRGT) may be programmed to be anyspecific magnitude depending upon the LED light source 102.

To control the intensity of the LED light source 102 during the currentload control mode, the control circuit 240 may control the LED drivecircuit 230 to adjust the amount of power delivered to the LED lightsource 102 using the PWM dimming technique, the PFM dimming technique,and/or the CCR dimming technique. Using the PWM dimming technique, thecontrol circuit 240 may control the peak magnitude I_(PK) of the loadcurrent I_(LOAD) through the LED light source 102 to the target loadcurrent I_(TRGT). Using the PWM dimming technique, the control circuit240 may pulse-width modulate the load current I_(LOAD) to dim the LEDlight source 102 and achieve the target load current I_(TRGT). Forexample, the LED drive circuit 230 may control (i.e., adjust) a dutycycle DC_(ILOAD) of the load current I_(LOAD) in response to a dutycycle DC_(DIM) of a dimming control signal V_(DIM)provided by thecontrol circuit 240. Further, when using the PWM dimming technique, theLED drive circuit 230 may maintain a frequency f_(ILOAD) of the loadcurrent I_(LOAD) in response to a frequency f_(DIM) of the dimmingcontrol signal V_(DIM) provided by the control circuit 240. Theintensity of the LED light source 102 may be dependent upon the dutycycle DC_(ILOAD) and the frequency f_(ILOAD) of the pulse-widthmodulated load current I_(LOAD).

Using the PFM dimming technique, the control circuit 240 may control thepeak magnitude I_(PK) of the load current I_(LOAD) through the LED lightsource 102 to the target load current I_(TRGT). Using the PFM dimmingtechnique, the control circuit 240 may pulse frequency modulate the loadcurrent I_(LOAD) to dim the LED light source 102 and achieve the targetload current I_(TRGT). For example, the LED drive circuit 230 maycontrol (i.e., adjust) a frequency f_(ILOAD) of the load currentI_(LOAD) in response to a frequency f_(DIM) of a dimming control signalV_(DIM) provided by the control circuit 240. Further, when using the PFMdimming technique, the LED drive circuit 230 may maintain the duty cycleDC_(ILOAD) of the load current I_(LOAD) in response to a duty cycleDC_(DIM) of the dimming control signal V_(DIM) provided by the controlcircuit 240. The intensity of the LED light source 102 may be dependentupon the duty cycle DC_(ILOAD) and the frequency f_(ILOAD) of thepulse-width modulated load current I_(LOAD).

Using the CCR technique, the control circuit 240 may not pulse-widthmodulate or pulse-frequency modulate the load current I_(LOAD). Usingthe CCR technique, the control circuit 240 may adjust the magnitude ofthe target load current I_(TRGT) so as to adjust the average magnitudeI_(AVE) of the load current I_(LOAD) through the LED light source 102.The average magnitude I_(AVE) of the load current I_(LOAD) through theLED light source 102 may be equal to the peak magnitude _(PK) of theload current I_(LOAD) in the CCR dimming mode.

During the voltage load control mode, the LED drive circuit 230 mayregulate the DC voltage of the load voltage V_(LOAD) across the LEDlight source 102 to a target load voltage V_(TRGT). The target loadvoltage V_(TRGT) may be stored in the memory 270. The target loadvoltage V_(TRGT) may be programmed to be any specific magnitudedepending upon the LED light source 102. The control circuit 240 may dimthe LED light source 102 using the PWM dimming technique and/or the PFMdimming technique during the voltage load control mode. For example,using the PWM dimming technique, the control circuit 240 may adjust aduty cycle DC_(VLOAD) of the load voltage V_(LOAD) in response to a dutycycle DC_(DIM) of the dimming control signal V_(DIM) to dim the LEDlight source 102. Using the PFM dimming technique, the control circuit240 may adjust the frequency f_(ILOAD) of the load voltage V_(LOAD) inresponse to a frequency f_(DIM) of the dimming control signal V_(DIM) todim the LED light source 102. An example of a configuration procedurefor the LED driver 200 is described in greater detail in U.S. patentapplication Ser. No. 12/813,989, filed Jun. 11, 2010, entitledCONFIGURABLE LOAD CONTROL DEVICE FOR LIGHT-EMITTING DIODE LIGHT SOURCES,the entire disclosure of which is hereby incorporated by reference.

FIG. 3A is a schematic diagram of an example of a flyback converter andan LED drive circuit. A flyback converter 320 may comprise a flybacktransformer 310, a field-effect transistor (FET) Q312, a diode D314, aresistor R316, a resistor R318, a flyback control circuit 322, a filtercircuit 324, an optocoupler circuit 326, and/or a feedback resistorR328. An LED drive circuit 330 may comprise a regulation field-effecttransistor (FET) Q332, a filter circuit 334, an amplifier circuit 336, agate resistor R338, a feedback circuit 342, a dimming FET Q350, a sampleand hold circuit (SHC) 360, and/or an overvoltage protection circuit370. The flyback converter 320 may be an example of the buck-boostflyback converter 220 of FIG. 2. The LED drive circuit 330 may be anexample of the LED drive circuit 230 of FIG. 2. As such, the LED driver100 of FIG. 1 and/or the LED driver 200 of FIG. 2 may comprise theflyback converter 320 and/or the LED drive circuit 330.

The flyback transformer 310 may comprise a primary winding and asecondary winding. The primary winding may be coupled in series with thefield-effect transistor (FET) Q312. Although illustrated as thefield-effect transistor (FET) Q312, the primary winding of the flybacktransformer 310 may be coupled in series with any flyback switchingtransistor or other suitable semiconductor switch. The secondary windingof the flyback transformer 310 may be coupled to the bus capacitorC_(BUS) via the diode D314. The bus voltage feedback signal V_(BUS-FB)may be generated by a voltage divider comprising the resistors R316,R318 coupled across the bus capacitor C_(BUS).

The flyback control circuit 322 may receive the bus voltage controlsignal V_(BUS-CNTL) from the control circuit 240, for example, via thefilter circuit 324 and the optocoupler circuit 326. The filter circuit324 and the optocoupler circuit 326 may provide electrical isolationbetween the flyback converter 320 and the control circuit 240. Theflyback control circuit 322 may comprise, for example, part numberTDA4863, manufactured by Infineon Technologies. The filter circuit 324may generate a filtered bus voltage control signal V_(BUS-F) using thebus voltage control signal V_(BUS-CNTL). For example, the filter circuit324 may comprise a two-stage resistor-capacitor (RC) filter forgenerating the filtered bus voltage control signal V_(BUS-F). Thefiltered bus voltage control signal V_(BUS-F) may comprise a DCmagnitude dependent upon the duty cycle DC_(BUS) of the bus voltagecontrol signal V_(BUS-CNTL). The flyback control circuit 322 may receivea control signal representative of the current through the FET Q312 fromthe feedback resistor R328, which is coupled in series with the FETQ312.

The flyback control circuit 322 may control the FET Q312 to selectivelyconduct current through the flyback transformer 310 to generate the busvoltage V_(BUS). The flyback control circuit 322 may render the FET Q312conductive and non-conductive at a high frequency (e.g., approximately150 kHz or less), for example, to control the magnitude of the busvoltage V_(BUS) in response to the DC magnitude of the filtered busvoltage control signal V_(BUS-F) and the magnitude of the currentthrough the FET Q312. For example, the control circuit 240 may increasethe duty cycle DC_(BUS) of the bus voltage control signal V_(BUS-CNTL)such that the DC magnitude of the filter bus voltage control signalV_(BUS-F) increases in order to decrease the magnitude of the busvoltage V_(BUS). The control circuit 240 may decrease the duty cycleDC_(BUS) of the bus voltage control signal V_(BUS-CNTL) to increase themagnitude of the bus voltage V_(BUS). The filter circuit 324 may providea digital-to-analog conversion for the control circuit 240 (i.e., fromthe duty cycle D_(BUS) of the bus voltage control signal V_(BUS-CNTL) tothe DC magnitude of the filtered bus voltage control signalV_(BUS-CNTL)). The control circuit 240 may comprise a digital-to-analogconverter (DAC) for generating (e.g., directly generating) the busvoltage control signal V_(BUS-CNTL) having an appropriate DC magnitudefor controlling the magnitude of the bus voltage V_(BUS).

FIG. 3B is a schematic diagram of an example of the LED drive circuit ofFIG. 3A. The LED drive circuit 330 may comprise the regulationfield-effect transistor (FET) Q332, the filter circuit 334, theamplifier circuit 336, the gate resistor R338, the feedback circuit 342,the dimming FET Q350, the sample and hold circuit 360, and/or theovervoltage protection circuit 370. The feedback circuit 342 maycomprise a feedback resistor R344, a filter circuit 346, and/or anamplifier circuit 348. The sample and hold circuit 360 may comprise aFET Q361, a capacitor C362, a resistor R363, a resistor R364, a FETQ365, a resistor R366, and/or a resistor R367. The overvoltageprotection circuit 370 may comprise a comparator U371, a resistor R372,a resistor R373, a resistor R374, a resistor R375, a filtering capacitorC376, a resistor R378, and/or a resistor R379.

The LED drive circuit 330 may comprise a linear regulator (i.e., acontrollable-impedance circuit) including the regulation field-effecttransistor (FET) Q332 coupled in series with the LED light source 102for conducting the load current I_(LOAD). Although illustrated as theFET Q332, the LED drive circuit 330 may comprise any power semiconductorswitch coupled in series with the LED light source 102 for conductingthe load current I_(LOAD). The regulation FET Q332 may comprise abipolar junction transistor (BJT), an insulated-gate bipolar transistor(IGBT), or any suitable transistor. The peak current control signalV_(IPK) provided by the control circuit 240 may be coupled to the gateof the regulation FET Q332 through the filter circuit 334, the amplifiercircuit 336, and the gate resistor R338. The control circuit 240 maycontrol the duty cycle DC_(IPK) of the peak current control signalV_(IPK) to control the peak magnitude I_(PK) of the load currentI_(LOAD) conducted through the LED light source 102 to the target loadcurrent I_(TRGT).

The filter circuit 334 (e.g., a two-stage RC filter) may providedigital-to-analog conversion for the control circuit 240, for example,by generating a filtered peak current control signal V_(IPK-F). Thefiltered peak current control signal V_(IPK-F) may have a DC magnitudedependent upon the duty cycle DC_(IPK) of the peak current controlsignal V_(IPK) and may be representative of the magnitude of the targetload current I_(TRGT). The control circuit 240 may comprise a DAC forgenerating (e.g., directly generating) the peak current control signalV_(IPK) having an appropriate DC magnitude for controlling the peakmagnitude I_(PK) of the load current I_(LOAD). The amplifier circuit 336may generate an amplified peak current control signal V_(IPK-A). Theamplifier circuit 336 may provide the amplified peak current controlsignal V_(IPK-A) to the gate of the regulation transistor Q332 throughthe resistor R338, such that a drive signal at the gate of theregulation transistor Q332, e.g., a gate voltage V_(IPK-G), has amagnitude dependent upon the target load current I_(TRGT). The amplifiercircuit 336 may comprise a standard non-inverting operational amplifiercircuit having, for example, a gain α of approximately three.

The feedback resistor R344 of the feedback circuit 342 may be coupled inseries with the regulation FET Q332, for example, such that the voltagegenerated across the feedback resistor is representative of themagnitude of the load current I_(LOAD). For example, the feedbackresistor R344 may have a resistance of approximately 0.0375 Ω. Thefilter circuit 346 (e.g., a two-stage RC filter) of the feedback circuit342 may be coupled between the feedback resistor R344 and the amplifiercircuit 348 (e.g., a non-inverting operational amplifier circuit havinga gain α of approximately 20). The amplifier circuit 348 may have avariable gain, which for example, may be controlled by the controlcircuit 240 and could range between approximately 1 and 1000. Theamplifier circuit 348 may generate the load current feedback signalV_(ILOAD). The amplifier circuit 348 may provide the load currentfeedback signal V_(ILOAD) to the control circuit 240. The load currentfeedback signal V_(ILOAD) may be representative of an average magnitudeI_(AVE) of the load current I_(LOAD), e.g.,I _(AVE) =V _(ILOAD)/(β·R _(FB)),   (Equation 1)wherein R_(FB) is the resistance of the feedback resistor R344. Examplesof other feedback circuits for the LED drive circuit 330 are describedin greater detail in U.S. patent application Ser. No. 12/814,026, filedJun. 11, 2010, entitled CLOSED-LOOP LOAD CONTROL CIRCUIT HAVING A WIDEOUTPUT RANGE, the entire disclosure of which is hereby incorporated byreference.

When operating in the current load control mode, the control circuit 240may control the regulation FET Q332 to operate in the linear region,such that the peak magnitude I_(PK) of the load current I_(LOAD) isdependent upon the DC magnitude of the gate voltage V_(IPK-G) at thegate of the regulation FET Q332. In other words, the regulation FET Q332may provide a controllable-impedance in series with the LED light source102. If the magnitude of the regulator voltage V_(REG) drops too low,the regulation FET Q332 may be driven into the saturation region, suchthat the regulation FET Q332 becomes fully conductive and the controlcircuit 240 is no longer able to control the peak magnitude I_(PK) ofthe load current I_(LOAD). Therefore, the control circuit 240 may adjustthe magnitude of the bus voltage V_(BUS) to prevent the magnitude of theregulator voltage V_(REG) from dropping below a minimum regulatorvoltage threshold V_(REG-MIN) (e.g., approximately 0.4 volts). Inaddition, the control circuit 240 may adjust the magnitude of the busvoltage V_(BUS) to control the magnitude of the regulator voltageV_(REG) to be less a maximum regulator voltage threshold V_(REG-MAX)(e.g., approximately 0.6 volts), for example, to prevent the powerdissipated in regulation FET Q332 from becoming too large, thusincreasing the total efficiency of the LED driver (e.g., the LED driver100, the LED driver 200, and/or the like). Since the regulator voltageV_(REG) may have some ripple (e.g., which may be due to the ripple ofthe bus voltage V_(BUS)), the control circuit 240 may determine theminimum value of the regulator voltage V_(REG) during a period of timeand to compare this minimum value of the regulator voltage V_(REG) tothe regulator voltage threshold V_(REG-MIN) and the maximum regulatorvoltage threshold V_(REG-MAX).

When operating in the voltage load control mode, the control circuit 240may drive the regulation FET Q332 into the saturation region, forexample, such that the magnitude of the load voltage V_(LOAD) isapproximately equal to the magnitude of the bus voltage V_(BUS) (e.g.,minus the small voltage drops due to the on-state drain-sourceresistance R_(DS-ON) of the FET regulation Q332 and the resistance ofthe feedback resistor R344).

The dimming FET Q350 of the LED drive circuit 330 may be coupled betweenthe gate of the regulation FET Q332 and circuit common The dimmingcontrol signal V_(DIM) from the control circuit 240 may be provided tothe gate of the dimming FET Q350. When the dimming FET Q350 is renderedconductive, the regulation FET Q332 may be rendered non-conductive. Whenthe dimming FET Q350 is rendered non-conductive, the regulation FET Q332may be rendered conductive.

While using the PWM dimming technique during the current load controlmode, the control circuit 240 may adjust the duty cycle DC_(DIM) of thedimming control signal V_(DIM) (e.g., to adjust the length of an on timet_(ON) that the regulation FET Q332 is conductive) to control when theregulation FET Q332 conducts the load current I_(LOAD) and to controlthe intensity of the LED light source 102. For example, the controlcircuit 240 may generate the dimming control signal V_(DIM) using aconstant frequency f_(DIM) (e.g., approximately in the range of 500-550Hz), such that the on time t_(ON) of the dimming control signal V_(DIM)is dependent upon the duty cycle DC_(DIM), i.e.,t _(ON)=(1−DC _(DIM))/f _(DIM). tm (Equation 2)As the duty cycle DC_(DIM) of the dimming control signal V_(DIM)increases, the duty cycle DC_(ITRGT), DC_(VTRGT) of the correspondingload current I_(LOAD) or load voltage V_(LOAD) decreases, and viceversa.

While using the PFM dimming technique during the current load controlmode, the control circuit 240 may adjust the frequency f_(DIM) of thedimming control signal V_(DIM) to control the frequency at which theregulation FET Q332 conducts the load current I_(LOAD) and to controlthe intensity of the LED light source 102. For example, the controlcircuit 240 may generate the dimming control signal V_(DIM) using aconstant on time t_(ON), such that the frequency f_(DIM) of the dimmingcontrol signal V_(DIM) is dependent upon the duty cycle DC_(DIM), i.e.,f _(DIM)=(1−DC _(DIM))/t _(ON).   (Equation 3)As the duty cycle DC_(DIM) of the dimming control signal V_(DIM)increases, the duty cycle DC_(ITRGT), DC_(VTRGT) of the correspondingload current I_(LOAD) or load voltage V_(LOAD) decreases, and viceversa.

When using the PWM dimming technique and/or the PFM dimming technique inthe current load control mode, the control circuit 240 may control thepeak magnitude I_(PK) of the load current I_(LOAD) in response to theload current feedback signal V_(ILOAD) to maintain the average magnitudeI_(AVE) of the load current I_(LOAD) constant (i.e., at the target lampcurrent I_(TRGT)). The control circuit 240 may calculate the peakmagnitude I_(PK)of the load current I_(LOAD) from the load currentfeedback signal V_(ILOAD) and the duty cycle DC_(DIM) of the dimmingcontrol signal V_(DIM), i.e.,I _(PK) =I _(AVE)/(1−DC _(DIM)).   (Equation 4)The load current feedback signal V_(ILOAD) may be representative of theaverage magnitude I_(AVE) of the load current I_(LOAD). When using theCCR dimming technique during the current load control mode, the controlcircuit 240 may maintain the duty cycle DC_(DIM) of the dimming controlsignal V_(DIM) at a high-end dimming duty cycle DC_(HE) (e.g.,approximately 0%, such that the FET Q332 is always conductive) and/ormay adjust the target load current I_(TRGT) (e.g., via the duty cycleDC_(IPK) of the peak current control signal V_(IPK)) to control theintensity of the LED light source 102.

The regulator voltage feedback signal V_(REG-FB) may be generated by thesample and hold circuit 360 of the LED drive circuit 330. The regulatorvoltage feedback signal V_(REG-FB) may be representative of theregulator voltage V_(REG) generated across the series combination of theregulation FET Q332 and the feedback resistor R344 when the regulationFET Q332 is conducting the load current I_(LOAD). The FET Q361 of thesample and hold circuit 360 may be coupled to the junction of the LEDlight source 102 and the regulation FET Q332. Although illustrated asthe FET Q361, the sample and hold circuit 360 may include any samplingtransistor. When the FET Q361 is rendered conductive, the capacitor C362may charge to approximately the magnitude of the regulator voltageV_(REG) through the resistor R363. The capacitor C362 may have acapacitance of approximately 1 μF. The resistor R363 may have aresistance of approximately 10 Ω. The capacitor C362 may be coupled tothe control circuit 240 through the resistor R364 for providing theregulator voltage feedback signal V_(REG-FB) to the control circuit 240.The resistor R364 may have a resistance of approximately 12.1 kΩ. Thegate of the FET Q361 may be coupled to circuit common through the FETQ365 and to the second isolated supply voltage V_(CC2) through theresistor R366. The resistor R366 may have a resistance of approximately20 kΩ. The gate of the second FET Q365 may be coupled to the thirdnon-isolated supply voltage V_(CC3) through the resistor R367. Theresistor R367 may have a resistance of approximately 10 kΩ.

The control circuit 240 may generate a sample and hold control signalV_(SH) that is operatively coupled to the control input (i.e., the gate)of the FET Q365 of the sample and hold circuit 360. The sample and holdcontrol signal V_(SH) may be coupled to the FET Q365 to render the FETQ361 conductive and non-conductive to controllably charge the capacitorC362 to the magnitude of the regulator voltage V_(REG). For example,when using the PWM dimming mode and/or the PFM dimming mode, the controlcircuit 240 may render the FET Q361 conductive during an on time t_(ON)(e.g., each on time t_(ON)) of the dimming control signal V_(DIM) (i.e.,when the dimming FET Q350 is non-conductive and the regulation FET Q332is conductive). When the FET Q361 is rendered conductive during the ontime t_(ON) of the dimming control signal V_(DIM), the regulator voltagefeedback signal V_(REG-FB) may be representative of the magnitude of theregulator voltage V_(REG) when the regulation FET Q332 is conducting theload current I_(LOAD). When the control circuit 240 is using the CCRdimming mode, the FET Q361 may be rendered conductive at all times.

The overvoltage protection circuit 370 of the LED drive circuit 330 maybe responsive to the magnitude of the bus voltage V_(BUS) and/or themagnitude of the regulator feedback voltage V_(REG-FB). The differencebetween the magnitudes of the bus voltage V_(BUS) and the regulatorfeedback voltage V_(REG-FB) may be representative of the magnitude ofthe load voltage V_(LOAD) across the LED light source 102. Thecomparator U371 of the overvoltage protection circuit 370 may have anoutput coupled to the gate of the regulation FET Q332 for rendering theFET non-conductive if the load voltage V_(LOAD) exceeds an overvoltagethreshold. The overvoltage protection circuit 370 may comprise aresistor divider that includes the resistors R372, R373. The resistordivider that includes the resistors R372, R373 may receive the regulatorfeedback voltage V_(REG-FB). The junction of the resistors R372, R373may be coupled to the non-inverting input of the comparator U371 throughthe resistor R374. The non-inverting input may be coupled to the thirdnon-isolated supply voltage V_(CC3) through the resistor R375 and/or tocircuit common through the filtering capacitor C376. The filteringcapacitor C376 may have a capacitance of approximately 10 μF.

The overvoltage protection circuit may comprise a resistor divider thatincludes the resistors 3478, 379. The resistor divider that includesresistors R378, R379 may be coupled between the bus voltage V_(BUS) andcircuit common. The junction of the resistors R378, R379 may be coupledto the inverting input of the comparator U371, such that, for example,the magnitude of the voltage at the non-inverting input of thecomparator U371 may be responsive to the regulator feedback voltageV_(REG-FB) and/or such that the magnitude of the voltage at theinverting input of the comparator U371 may be responsive to the busvoltage V_(BUS). The comparator U371 may operate to render theregulation FET Q332 non-conductive if the difference between themagnitudes of the bus voltage V_(BUS) and the regulator feedback voltageV_(REG-FB) exceeds the overvoltage threshold.

The resistances of the resistors R372, R373, R374, R375, R378, R379 ofthe overvoltage protection circuit 370 may be determined such that thevoltage at the non-inverting input of the comparator U371 isproportional to the magnitude of the regulator feedback voltageV_(REG-FB). Accordingly, the magnitude of the bus voltage V_(BUS) thatmay cause the voltage at the inverting input of the comparator U371 toexceed the voltage at the non-inverting input increases in proportionalto the magnitude of the regulator feedback voltage V_(REG-FB), such thatthe overvoltage threshold that the load voltage V_(LOAD) exceeds torender the regulation FET Q332 non-conductive remains approximatelyconstant as the magnitude of the regulator feedback voltage V_(REG-FB)changes. The resistances of the resistors R375, R374 may be greater thanthe resistances of the resistors R372, R373 to avoid loading theregulator feedback voltage V_(REG-FB).

FIG. 4A is a graph that illustrates an example of the relationshipbetween an on-time T_(ON) of a load current of an LED driver (e.g., theLED driver 100 of FIG. 1, the LED driver 200 of FIG. 2, and/or the like)and a target lighting intensity L_(TRGT) of an LED light source (e.g.,the LED light source 102 and/or the like). FIG. 4B is a graph thatillustrates an example of the relationship between a frequency f_(LOAD)of a load current of an LED driver (e.g., the LED driver 100 of FIG. 1,the LED driver 200 of FIG. 2, and/or the like) and a target lightingintensity L_(TRGT) of an LED light source (e.g., the LED light source102 and/or the like).

FIG. 5A is a graph that illustrates an example of the relationshipbetween an on-time T_(ON) of a load current of an LED driver (e.g., theLED driver 100 of FIG. 1, the LED driver 200 of FIG. 2, and/or the like)and a target lighting intensity L_(TRGT) of an LED light source (e.g.,the LED light source 102 and/or the like). FIG. 5B is a graph thatillustrates an example of the relationship between a frequency f_(LOAD)of a load current of an LED driver (e.g., the LED driver 100 of FIG. 1,the LED driver 200 of FIG. 2, and/or the like) and a target lightingintensity L_(TRGT) of an LED light source (e.g., the LED light source102 and/or the like). One or more of the embodiments described withrelation to FIGS. 4A, 4B, 5A, and/or 5B may be performed by an LEDdriver (e.g., the LED driver 100 of FIG. 1, the LED driver 200 of FIG.2, and/or the like) using a current control mode and/or a voltagecontrol mode.

The control circuit 240 may be configured to control the intensity ofthe LED light source 102 by pulse width modulating the load currentI_(LOAD) when the target intensity is above a predetermined thresholdand control the intensity of the LED light source 102 by pulse frequencymodulating the load current I_(LOAD) when the target intensity is belowthe predetermined threshold. The predetermined threshold may be, forexample, a low-end intensity L_(LE) (e.g., 1%) as shown in FIGS. 4A-4Band FIGS. 5A-5B. Pulse width modulating the load current I_(LOAD) maycomprise maintaining a frequency f_(LOAD) of the load current I_(LOAD)constant and adjusting an on time T_(ON) of the load current I_(LOAD).Pulse frequency modulating the load current I_(LOAD) may comprisemaintaining the on time T_(ON) of the load current I_(LOAD) constant andadjusting the frequency f_(LOAD) of the load current I_(LOAD).

When the LED driver is operating in the PWM dimming mode, the controlcircuit 240 may adjust the duty cycle DC_(ILOAD) of the pulse-widthmodulated load current I_(LOAD) to dim the LED light source 102 betweenthe high-end intensity L_(HE) (e.g., approximately 100%) and the low-endintensity L_(LE) (e.g., approximately 1%) in response to thephase-control signal V_(PC). For example, the control circuit 240 mayrender the dimming FET Q350 conductive for an on time T_(ON) andnon-conductive for an off time T_(OFF) during a period (e.g., eachperiod) T_(PWM) of the pulse-width modulated load current I_(LOAD). Thecontrol circuit 240 may hold a frequency f_(LOAD) of the pulse-widthmodulated load current I_(LOAD) constant at a normal PWM frequencyf_(NORM) (e.g., approximately in the range of 500-550 Hz) and may adjustthe length of the on time T_(ON) to dim the LED light source 102 betweenthe high-end intensity L_(HE) and the low-end intensity L_(LE), forexample, as shown in FIGS. 4A-4B and FIGS. 5A-5B. For example, thelength of the on time T_(ON) may be controlled between a maximum on timeT_(MAX) (e.g., approximately 1.8 msec) corresponding to the high-endintensity L_(HE) (e.g., the duty cycle DC_(ILOAD) may equalapproximately 100%) of the LED light source 102 and a minimum on timeT_(MIN) (e.g., approximately 18 μsec) corresponding to the low-endintensity L_(LE) (e.g., the duty cycle DC_(ILOAD) may equalapproximately 1%) of the LED light source 102.

The LED driver may adjust (e.g., fade) the intensity of the LED lightsource 102 from the present intensity L_(PRES) to off (e.g., 0%) over afade time period T_(FADE). When fading the intensity of the LED lightsource 102 to off, the control circuit 240 may adjust the intensity ofthe LED light source 102 below the low-end intensity L_(LE) (e.g., 1%),for example, to a minimum intensity L_(MIN), to an ultra-low minimumintensity L_(MIN-UL), and/or to off. Hardware limitations of the controlcircuit 240 (e.g., a minimum pulse width that may be generated by thecontrol circuit) may prevent the length of the on time T_(ON) of thepulse-width modulated load current I_(LOAD) from being adjusted belowthe minimum on time T_(MIN), for example, when the frequency f_(LOAD) ofthe pulse-width modulated load current I_(LOAD) is at the normal PWMfrequency f_(NORM).

The control circuit 240 may adjust the intensity of the LED light source102 below the low-end intensity L_(LE) by pulse frequency modulating theload current I_(LOAD). For example, the control circuit 240 may adjustthe intensity of the LED light source 102 below the low-end intensityL_(LE) to the minimum intensity L_(MIN) by the maintaining the length ofthe on time T_(ON) constant at the minimum on time T_(MIN) anddecreasing the frequency f_(LOAD) of the pulse-width modulated loadcurrent I_(LOAD), for example, as shown in FIGS. 4A-4B and 5A-5B. Thecontrol circuit 240 may decrease the intensity of the LED light source102 from the low-end intensity L_(LE) to the minimum intensity L_(MIN)(e.g., approximately 0.1%) by decreasing the frequency f_(LOAD) from thenormal PWM frequency f_(NORM) to a minimum PWM frequency f_(MIN) (e.g.,approximately 120 Hz). As such, the control circuit 240 may adjust theintensity of the LED light source 102 by adjusting the length of the ontime T_(ON) and maintaining the frequency f_(LOAD) when the targetintensity L_(TRGT) is greater than the low-end intensity L_(LE), and byadjusting the frequency f_(LOAD) and maintaining the on time T_(ON) whenthe target intensity L_(TRGT) is less than the low-end intensity L_(LE).In one or more embodiments, the control circuit 240 may decrease theintensity of the LED light source 102 from the low-end intensity L_(LE)to off by decreasing the frequency f_(LOAD), for example, from thenormal PWM frequency f_(NORM) to the minimum PWM frequency f_(MIN).

The control circuit 240 may control the intensity of the LED lightsource 102 below the minimum intensity L_(MIN) to an ultra-low minimumintensity L_(MIN-UL), for example, as shown in FIGS. 5A-5B. The controlcircuit 240 may control the intensity of the LED light source 102 belowthe minimum intensity L_(MIN) to an ultra-low minimum intensityL_(MIN-UL) by pulse width modulating the load current. For example, thecontrol circuit 240 may maintain the frequency f_(LOAD) of the loadcurrent I_(LOAD) constant at the minimum PWM frequency f_(MIN) (e.g.,approximately 120 Hz) and decrease the on time T_(ON) below the minimumon time T_(MIN). For example, the control circuit 240 may decrease theon time T_(ON) from the minimum on time T_(MIN) to an ultra-low minimumon time T_(MIN-UL) while maintaining the frequency f_(LOAD) constant atthe minimum PWM frequency f_(MIN) to control the intensity of the LEDlight source 102 below the minimum intensity L_(MIN) to an ultra-lowminimum intensity L_(MIN-UL). For instance, the control circuit may dimthe LED light source to off by pulse width modulating the load current(i.e., the ultra-low minimum intensity may be 0% intensity). In suchexamples, the minimum PWM frequency may be decreased below 120 Hz. Thecontrol circuit 240 may decrease the on time T_(ON) until the hardwarelimitations of the control circuit 240 prevent the on time T_(ON) frombeing decreased any further.

The control circuit 240 may be configured to dim the LED light source102 to off. The control circuit 240 may be configured to control theintensity of the LED light source 102 from the predetermined thresholdto off by pulse frequency modulating the load current I_(LOAD). Thecontrol circuit 240 may be configured to pulse width modulate the loadcurrent I_(LOAD) when the target intensity is below the minimumintensity L_(MIN). As such, the control circuit 240 may be configured tocontrol the intensity of the LED light source 102 from the minimumintensity L_(MIN) to off by pulse width modulating the load currentI_(LOAD).

In one or more embodiments, the control circuit 240 may control theintensity of the LED light source 102 by decreasing the magnitude of theDC bus voltage V_(BUS). For example, the control circuit 240 may beconfigured to control the intensity of the LED light source 102 belowthe minimum intensity level L_(MIN) by decreasing the magnitude of theDC bus voltage V_(BUS). The control circuit may be configured tomaintain a frequency f_(LOAD) of the load current I_(LOAD) constant(e.g., at the minimum PWM frequency f_(MIN)), maintain an on time T_(ON)of the load current I_(LOAD) constant (e.g., at the minimum on timeT_(MIN) or at the ultra-low minimum on time T_(MIN-UL)), and decrease amagnitude of the DC bus voltage V_(BUS) when the target intensityL_(TRGT) is below the minimum intensity L_(MIN). For example, controlcircuit may control the intensity of the LED light source 102 to off bydecreasing the magnitude of the DC bus voltage V_(BUS).

The high-end intensity L_(HE) may be approximately 100%. The low-endintensity L_(LE) may be approximately 1%. The minimum intensity L_(MIN)may be approximately in the range of 0.1-1%. The ultra-low minimumintensity L_(MIN-UL) may be approximately in the range of 0-0.1%. Forexample, the ultra-low minimum intensity L_(MIN-UL) may be 0% (i.e.,off). The maximum on time T_(MAX) may be approximately 1.8 msec. Theminimum on time T_(MIN) may be approximately 18 μsec. The ultra-lowminimum on time T_(MIN-UL) may be approximately 1 μsec. The normal PWMfrequency f_(NORM) may be approximately in the range of 500-550 Hz. Theminimum PWM frequency f_(MIN) may be approximately in the range of120-15 0Hz.

Although illustrated in FIGS. 4A-4B and FIGS. 5A-5B as controlling thelength of the on time T_(ON) of the load current I_(LOAD) between thehigh-end intensity L_(HE) and the minimum intensity L_(MIN) andcontrolling the frequency f_(LOAD) of the load current I_(LOAD) betweenthe minimum intensity L_(MIN) and the ultra-low minimum intensityL_(MIN-UL), the control circuit 240 may be configured to control theintensity of the LED light source 102 by pulse width modulating the loadcurrent I_(LOAD) when the target intensity is within a first intensityrange and control the intensity of the LED light source 102 by pulsefrequency modulating the load current I_(LOAD) when the target intensityis within a second intensity range. The first intensity range may begreater than or less than the second intensity range. Further, thecontrol circuit 240 may be configured to control the intensity of theLED light source 102 by pulse width modulating the load current I_(LOAD)when the target intensity is within a third intensity range. The thirdintensity range may be below a known operating range of the LED lightsource 102. As such, the control circuit 240 may control the LED lightsource 102 by adjusting a first parameter (e.g., on time T_(ON) of theload current I_(LOAD)) to a control point that produces a known,reliable response of the LED light source 102 (e.g., the low-endintensity L_(LE)), adjusting a second parameter (e.g., the frequencyf_(LOAD) of the load current I_(LOAD)) to a second control point thatmay or may not produce a known reliable response of the LED light source102, and adjusting the first parameter past the second control point,which may produce an unknown and potentially unreliable response of theLED light source 102. However, this may be acceptable because thecontrol circuit 240 may be fading the LED light source 102 to off.

The control circuit 240 may be configured to receive a command andcontrol (e.g., dim) the intensity of the LED light source 102 below thefirst intensity range and below the second intensity range to off. Forexample, the load control circuit may be configured to control theintensity of the LED light source 102 below the second intensity rangeto off by pulse width modulating and/or pulse frequency modulating theload current I_(LOAD). The load control circuit may be configured tocontrol the intensity of the LED light source 102 below the firstintensity range and below the second intensity range to off bymaintaining the frequency f_(LOAD) of the load current I_(LOAD)constant, maintaining the on time T_(ON) of the load current I_(LOAD)constant, and decreasing the magnitude of the DC bus voltage V_(BUS).

The control circuit 240 may control the length of the on time T_(ON)and/or the frequency f_(LOAD) of the load current ILOAD to adjust theintensity of the LED light source 102 between the minimum intensityL_(MIN) (e.g., 0.1%) and the high-end intensity L_(HE) (e.g., 100%)during, for example, normal operation of the LED driver (i.e., not onlywhen the LED driver is fading the intensity of the LED light source tooff).

One or more of the embodiments described herein (e.g., as performed by aload control device) may be used to decrease the intensity of a lightingload and/or increase the intensity of the lighting load. For example,one or more embodiments described herein may be used to adjust theintensity of the lighting load from on to off, off to on, from a higherintensity to a lower intensity, and/or from a lower intensity to ahigher intensity. For example, although described as adjusting theintensity of the LED light source 102 from the present intensityL_(PRES) to off (e.g., 0%), the LED driver may adjust (e.g., fade) theintensity of the LED light source 102 from off (e.g., 0%) to a targetintensity L_(TRGT) (e.g., an intensity between an ultra-low minimumintensity L_(MIN-UL) and a high-end intensity L_(HE)) to over the fadetime period T_(FADE) (e.g., in accordance with FIGS. 4A, 4B, 5A, and/or5B).

Although described with reference to an LED driver, one or moreembodiments described herein may be used with other load controldevices. For example, one or more of the embodiments described hereinmay be performed by a variety of load control devices that areconfigured to control of a variety of electrical load types, such as,for example, a LED driver for driving an LED light source (e.g., an LEDlight engine); a screw-in luminaire including a dimmer circuit and anincandescent or halogen lamp; a screw-in luminaire including a ballastand a compact fluorescent lamp; a screw-in luminaire including an LEDdriver and an LED light source; a dimming circuit for controlling theintensity of an incandescent lamp, a halogen lamp, an electroniclow-voltage lighting load, a magnetic low-voltage lighting load, oranother type of lighting load; an electronic switch, controllablecircuit breaker, or other switching device for turning electrical loadsor appliances on and off; a plug-in load control device, controllableelectrical receptacle, or controllable power strip for controlling oneor more plug-in electrical loads (e.g., coffee pots, space heaters,other home appliances, and the like); a motor control unit forcontrolling a motor load (e.g., a ceiling fan or an exhaust fan); adrive unit for controlling a motorized window treatment or a projectionscreen; motorized interior or exterior shutters; a thermostat for aheating and/or cooling system; a temperature control device forcontrolling a heating, ventilation, and air conditioning (HVAC) system;an air conditioner; a compressor; an electric baseboard heatercontroller; a controllable damper; a humidity control unit; adehumidifier; a water heater; a pool pump; a refrigerator; a freezer; atelevision or computer monitor; a power supply; an audio system oramplifier; a generator; an electric charger, such as an electric vehiclecharger; and an alternative energy controller (e.g., a solar, wind, orthermal energy controller). A single control circuit may be coupled toand/or adapted to control multiple types of electrical loads in a loadcontrol system.

What is claimed is:
 1. A load control device for controlling anintensity of a lighting load, the load control device comprising: a loadregulation circuit configured to control a magnitude of a load currentconducted through the lighting load; and a control circuit operativelycoupled to the load regulation circuit for pulse width modulating orpulse frequency modulating the load current to control the intensity ofthe lighting load to a target intensity; wherein the control circuit isconfigured to control the intensity of the lighting load by pulse widthmodulating the load current when the target intensity is above apredetermined threshold and control the intensity of the lighting loadby pulse frequency modulating the load current when the target intensityis below the predetermined threshold.
 2. The load control device ofclaim 1, wherein the control circuit is configured to pulse widthmodulate the load current when the target intensity is below a minimumintensity that is below the predetermined threshold.
 3. The load controldevice of claim 2, wherein the control circuit is configured to controlthe intensity of the lighting load from the minimum intensity to off bypulse width modulating the load current.
 4. The load control device ofclaim 1, wherein pulse width modulating the load current comprisesmaintaining a frequency of the load current constant and adjusting an ontime of the load current, and wherein pulse frequency modulating theload current comprises maintaining the on time of the load currentconstant and adjusting the frequency of the load current.
 5. The loadcontrol device of claim 1, wherein the control circuit is configured tomaintain a frequency of the load current at a normal pulse widthmodulation (PWM) frequency and adjust an on time of the load currentbetween a maximum on time and a minimum on time when the targetintensity is above the predetermined threshold; and wherein the controlcircuit is configured to maintain the on time of the load current at theminimum on time and adjust the frequency of the load current between thenormal PWM frequency and a minimum PWM frequency when the targetintensity is below the predetermined threshold.
 6. The load controldevice of claim 1, wherein the predetermined threshold is a low-endintensity, and the control circuit is configured to: maintain afrequency of the load current at a normal pulse width modulation (PWM)frequency and adjust an on time of the load current between a maximum ontime and a minimum on time when the target intensity is between ahigh-end intensity and the low-end intensity; maintain the on time ofthe load current at the minimum on time and adjust the frequency of theload current between the normal PWM frequency and a minimum PWMfrequency when the target intensity is between the low-end intensity anda minimum intensity; and maintain the frequency of the load current atthe minimum PWM frequency and adjust the on time of the load currentbetween the minimum on time and an ultra-low minimum on time when thetarget intensity is between the minimum intensity and an ultra-lowminimum intensity.
 7. The load control device of claim 1, wherein thecontrol circuit is configured to control the intensity of the lightingload from the predetermined threshold to off by pulse frequencymodulating the load current.
 8. The load control device of claim 1,wherein the control circuit is configured to control the intensity ofthe lighting load by pulse width modulating the load current when thetarget intensity is within a first intensity range and control theintensity of the lighting load by pulse frequency modulating the loadcurrent when the target intensity is within a second intensity range;and wherein the control circuit is configured to receive a command andcontrol the intensity of the lighting load below the first intensityrange and below the second intensity range to off.
 9. The load controldevice of claim 1, further comprising: a power converter circuitconfigured to receive a rectified AC voltage and to generate a DC busvoltage; wherein the load regulation circuit is configured to receivethe DC bus voltage, the control circuit configured to maintain afrequency of the load current constant, maintain an on time of the loadcurrent constant, and decrease a magnitude of the DC bus voltage whenthe target intensity is below a minimum intensity, wherein the minimumintensity is below the predetermined threshold.
 10. The load controldevice of claim 1, wherein the lighting load comprises an LED lightsource and the load regulation circuit comprises an LED drive circuit.11. A load control device for controlling an intensity of a lightingload, the load control device comprising: a load regulation circuitconfigured to control a magnitude of a load current conducted throughthe lighting load; and a control circuit operatively coupled to the loadregulation circuit for adjusting the magnitude of the load current tocontrol the intensity of the lighting load to a target intensity;wherein the control circuit is configured to control the intensity ofthe lighting load by maintaining a frequency of the load currentconstant and adjusting an on time of the load current when the targetintensity is above a predetermined threshold, and maintaining the ontime of the load current constant and adjusting the frequency of theload current when the target intensity is below the predeterminedthreshold.
 12. The load control device of claim 11, wherein the controlcircuit is configured to maintain the frequency of the load current at anormal pulse width modulation (PWM) frequency and adjust the on time ofthe load current between a maximum on time and a minimum on time whenthe target intensity is above the predetermined threshold; and whereinthe control circuit is configured to maintain the on time of the loadcurrent at the minimum on time and adjust the frequency of the loadcurrent between the normal PWM frequency and a minimum PWM frequencywhen the target intensity is below the predetermined threshold.
 13. Amethod for controlling an intensity of a lighting load, the methodcomprising: controlling a magnitude of a load current conducted throughthe lighting load; receiving a target intensity for the lighting load;pulse width modulating the load current to control the intensity of thelighting load to the target intensity when the target intensity is abovea predetermined threshold; and pulse frequency modulating the loadcurrent to control the intensity of the lighting load to the targetintensity when the target intensity is below the predeterminedthreshold.
 14. The method of claim 13, wherein pulse width modulatingthe load current comprises maintaining a frequency of the load currentconstant and adjusting an on time of the load current, and wherein pulsefrequency modulating the load current comprises maintaining the on timeof the load current constant and adjusting the frequency of the loadcurrent.
 15. The method of claim 13, wherein the method comprising:maintaining a frequency of the load current at a normal pulse widthmodulation (PWM) frequency and adjusting an on time of the load currentbetween a maximum on time and a minimum on time when the targetintensity is above the predetermined threshold; and maintaining the ontime of the load current at the minimum on time and adjusting thefrequency of the load current between the normal PWM frequency and aminimum PWM frequency when the target intensity is below thepredetermined threshold.
 16. The method of claim 13, wherein the methodcomprises: pulse width modulating the load current when the targetintensity is below a minimum intensity, wherein the minimum intensity isbelow the predetermined threshold.
 17. The method of claim 16, whereinthe method comprises: controlling the intensity of the lighting loadfrom the minimum intensity to off by pulse width modulating the loadcurrent.
 18. The method of claim 13, wherein the predetermined thresholdis a low-end intensity and wherein the method comprises: maintaining afrequency of the load current at a normal pulse width modulation (PWM)frequency and adjusting an on time of the load current between a maximumon time and a minimum on time when the target intensity is between ahigh-end intensity and the low-end intensity; maintaining the on time ofthe load current at the minimum on time and adjusting the frequency ofthe load current between the normal PWM frequency and a minimum PWMfrequency when the target intensity is between the low-end intensity anda minimum intensity; and maintaining the frequency of the load currentat the minimum PWM frequency and adjusting the on time of the loadcurrent between the minimum on time and an ultra-low minimum on timewhen the target intensity is between the minimum intensity and anultra-low minimum intensity.
 19. The method of claim 13, wherein themethod comprises: controlling the intensity of the lighting load fromthe predetermined threshold to off by pulse frequency modulating theload current.
 20. The method of claim 13, further comprising: receivinga rectified AC voltage; generating a DC bus voltage from the rectifiedAC voltage, wherein the magnitude of the load current conducted throughthe lighting load is controlled using the DC bus voltage; maintaining afrequency of the load current constant, maintaining an on time of theload current constant, and decreasing a magnitude of the DC bus voltagewhen the target intensity is below a minimum intensity, wherein theminimum intensity is below the predetermined threshold.